my %hash;
while (<DATA>) {
    chomp($_);
    my ($first, $second) = split(/s+/, $_);
    push(@{$hash{$first}}, $second);
}

foreach my $key (keys %hash) {
    print "The $key is: ", join(', ', @{$hash{$key}}), "\n";
}
[download]

should do what you want. The list references that are the value of the hash are instantiated autmagically if necessary.

Hope this helps, -gjb-

Update: Sorry, missed the uniqueness requirement. Since a number of good solutions have already been proposed I'll just give an alternative one. (I love sets.)

use Set::Scalar;

my %hash;
while (<DATA>) {
    chomp($_);
    my ($first, $second) = split(/s+/, $_);
    if (!exists $hash{$first}) {
        $hash{$first} = new Set::Scalar();
    }
    $hash{$first}->insert($second);
}

foreach my $key (keys %hash) {
    print "The Key is: $key\n\t",
          join("\n\t", $hash{$key}->members()),
          "\n";
}
[download]